Bring mixture to a boil, then reduce heat to a low simmer. Simmer (uncovered) for 45 - 60 minutes or until very thick. Stir frequently to avoid burning.

Add malt vinegar and prepared mustard to mixture. Return to a simmer and simmer uncovered until thick, or volume has reduced by 1/3. (20 - 30 minutes). Simmering too long will reduce the zing. One large deep pot will take longer to reduce the mixture than a small pot, use a smaller pot and reduce mixture in batches if necessary.